This invention relates to telescopic props and self-locking devices therefor.
The invention is particularly but not exclusively applicable to telescopic props for use in building construction, of the kind used, for example, to support overhead formwork in concrete pouring and other construction works.
The use of telescopic props to support concrete formwork is well known. The most common type of telescopic prop, commonly known as the "adjustable screw prop," has an inner and outer tube both of which are provided with diametrically opposite pairs of holes at intervals. A locking pin is inserted through aligned pairs of holes in the inner and outer tubes to lock the inner tube relatively to the outer tube at an overall length which approximates as closely as possible to the desired overall length. Final adjustment of the overall length of the telescopic prop is effected by means of a screw mechanism. In practice this screw adjustment may entail changing the overall length of the prop by a considerable amount, for example up to three inches, according to the distance between centres of adjacent pairs of holes along the inner tubes. Not only is such screw adjustment difficult and time-consuming, it is also greatly hindered when the threads of the adjusting screw have become damaged or clogged with cement, or corroded, as frequently occurs in practice.
Another disadvantage of the traditional type of telescopic prop referred to above is that the entire load on the prop is taken by the locking pin inserted through the aligned holes in the inner and outer tubes; the pin in consequence can become bent, or even sheared, and the holes in which the pin is placed can become elongated, making insertion and withdrawal of the pin extremely difficult. Moreover, since the locking pin is a separate item, it is apt to become mislaid when the telescopic prop is not in use. Another disadvantage of this type of telescopic prop is that a range of different sizes of prop is usually necessary to cater for different ranges of extension of the prop.
The present invention aims to provide a self-locking device for a telescopic prop which avoids the disadvantages associated with the use of a locking pin and which is readily adjustable to different heights, being locked automatically when loaded vertically in use.